1. Field of the Invention
This invention relates to a drilling tool suitable for use in the drilling of boreholes, for example for subsequent use in the extraction of oil and/or natural gas. In particular the invention relates to a drilling tool whereby borehole regions of increased director can be formed.
2. Description of the Related Art
When drilling or finishing boreholes, it is sometimes required to provide regions of increased diameter. Obviously, the tools used in the formation of the increased diameter regions of the borehole must be capable of being passed through any smaller diameter regions of the borehole above the position at which the larger diameter region is to be formed. This places considerable design constraints on the tools that can be used in such applications.
It is known to use drilling tools having a number of movable components, movable to vary the diameter of the borehole drilled thereby. However, the number of movable parts can result in such tools being relatively complex, susceptible to failure as a result of parts thereof becoming jammed, and also require associated control systems to be provided to control the operation thereof.
Another known type of tool suitable for use in such applications is a bi-center tool. Such a tool may include a pilot or mid-reamer region and a main reamer region located eccentrically to the mid-reamer region. In such an arrangement, when the tool is required to form a region of relatively large diameter, it is rotated about the axis of rotation of the mid-reamer region, while a weight on bit loading is applied. The mid-reamer region forms or finishes a bore region of relatively small diameter which is subsequently enlarged by the eccentric main reamer region, the mid-reamer region bearing against the wall of the bore and serving to guide the tool, reacting the side loadings applied due to the eccentric positioning of the main reamer region. Where smaller diameter regions are required, the tool is supported with the axis of rotation of the mid-reamer region located eccentric to the center of the borehole, the dimensions of the tool being such as to permit it is to be passed through smaller diameter regions of the borehole. Tools of this general type are described in, for example, U.S. Pat. No. 5,678,644 and US 2002/0104688.
The eccentric location of the main reamer region, as is necessary in a bi-center tool, results in the tool being out of balance, in use, in the manner outlined hereinbefore. The side loadings resulting form the tool being out-of-balance may result in, for example, the tool tending to tilt, and result in the tool being urged in directions other than that in which it is desired to extend the borehole. Also, the out-of-balance forces may result in the tool tending to rotate about an axis other than the intended axis of rotation of the mid-reamer region which can result in the part of the borehole being drilled by the main reamer region being of a different, for example smaller, diameter than desired. Further, the out-of-balance forces may result in certain parts of the tool being subject to excessive wear or result in damage thereto, in use. Obviously, these effects are disadvantageous and it is an object of the invention to provide a drilling tool in which the disadvantages set out hereinbefore are overcome or of reduced effect.
In the U.S. Pat. No. 5,678,644 arrangement, the drilling tool is designed in such a manner as to include penetration limiting means operable to limit the depth of penetration of at least some of the cutters provided on the tool. It is thought that limiting the depth of penetration in this way can serve to reduce tilting and whirling of the bit. However, limiting the depth of penetration in this manner only addresses issues arising from excessive penetration, and does not address instability arising from other factors. U.S. Pat. No. 5,678,644 further describes a design technique whereby the resultant cutting force of the pilot section of the bit and that of the reamer section of the bit can be substantially balanced in the sense that they are substantially oppositely directed and of substantially equal magnitude.